Gravity gradiometers have existed for many years and are used to measure variations in the gradient of the earth's gravitational field. Gravity gradiometers may be used in exploration for minerals and hydrocarbons, since deposits of these things in the earth, and variations in the underground structure containing the deposits, produce variations in gravity and in the gravitational gradients which if interpreted correctly can lead to valuable discoveries. The ability to operate a gravity gradiometer in a moving vehicle is desirable, since doing so can greatly decrease the amount of time needed to carry out a survey of a given site.
The variations in the gravity gradients which must be measured are extremely small in magnitude and therefore require very sensitive, low noise instruments with very repeatable response characteristics. Moreover, when the gravity gradiometer is mounted in a moving vehicle, the signals due to these gravity gradient changes are very small in comparison to the undesirable responses of the instrument produced by accelerations of the vehicle on which the instrument is mounted.
The performance of present commercially operating airborne gravity gradiometers is currently limited to an error level of about 5 Eotvos (1 Eo=gradient of 10−9 meters per second squared per meter, approximately 10−10 g per meter) at a signal averaging time of 6 seconds. Although this performance has been sufficient to hint at the potential usefulness of airborne gravity gradiometry, improvement to a performance level of 1 Eo averaged once per second is believed to be required for widespread successful application in mineral exploration.
A known form of gradiometer which has the laboratory demonstrated potential to provide this performance gain is the so-called orthogonal quadrupole responder (also referred to here as an OQR, and also known as the cross-component gravity gradiometer). In the OQR, two orthogonally oriented balance beams (also referred to here as mass quadrupoles), each being a body of elastic material with a distributed mass including a mass quadrupole moment, are attached to a housing using torsional springs, thus comprising quadrupole responders (also sometimes called angular accelerometers). The balance beams rotate differentially (in opposite directions) in response to changes in gravity gradients, but rotate in common mode (both in the same direction) in response to rotational acceleration motions of the vehicle. Thus, in principle, the OQR separates the weak gravity gradient signals from the much larger noise due to vehicle motions.
Early versions of a rotating OQR design have been disclosed by Weber, Zipoy and Forward in U.S. Pat. No. 3,722,284 (cf. FIG. 10 and associated discussion), and by Robert L. Forward, “Future lunar gravity measurements,” Earth, Moon, and Planets, Volume 22, No. 4 (1980) pp. 419-433, and by Lautzenhiser in U.S. Pat. No. 4,215,578. Ho Jung Paik, in “Superconducting tensor gravity gradiometer for satellite geodesy and inertial navigation,” The Journal of the Astronautical Sciences, Volume XXIX, No. 1, pp. 1-18, January-March 1981, presented a description of a Cross Component Gradiometer (discussion on p. 7, and FIG. 4), which is topologically equivalent to Forward's design, but which utilizes superconducting materials, pancake coils and SQUID detectors in order to achieve stable operation without needing to rotate. A later version also employing superconductive materials is disclosed by Van Kann and Buckingham in U.S. Pat. No. 5,668,315, and is described as an OQR by Van Kann et al., “Laboratory tests of a mobile superconducting gravity gradiometer”, Physica B, Volume 165 (1990) pp. 93-94. In Moody, Paik & Canavan, “Principle and performance of a superconducting angular accelerometer”, Review of Scientific Instruments, Volume 74, Issue 3 (2003) pp. 1310-1318, details of a built and tested superconducting angular accelerometer are described, a pair of which can be used to form an OQR gravity gradiometer.
Existing examples of OQR gravity gradiometers make use of cryogenic temperatures, both to permit the use of SQUID (superconductive quantum interference device) based detection of the quadrupole responders' motion, and to achieve almost perfectly elastic behavior in the torsional springs on which the mass quadrupoles are mounted. Van Kann and Buckingham describe one such OQR gravity gradiometer in U.S. Pat. No. 5,668,315. Another version is described in Moody, M. V. and Paik, H. J., “A superconducting gravity gradiometer for inertial navigation”, in Proc. IEEE 2004 Position Location and Navigation Symposium (PLANS 2004), April 2004, pp. 775-781. At temperatures significantly above cryogenic temperatures, including standard room temperature, all polycrystalline materials exhibit creep and hysteresis effects which would degrade instrument response repeatability (which is why conventional gravity meters are constructed of amorphous fused quartz, which exhibits much lower creep and hysteresis).
Cryogenic temperatures may be achieved by placing a superconducting gravity gradiometer in a cryostat cooled using a supply of liquid helium or other means of cooling. The necessity for liquid helium supply in the field, and the weight, size, and complexities associated with the liquid helium cryostat are undesirable features. Moreover, OQR-type gravity gradiometers currently use torsional springs which are in the form of a “microscopically” thick web joining the balance beams to the base. These webs are fragile, especially at higher temperatures similar to room temperature, and are prone to breaking. In addition, it is difficult to achieve requisite dimensional tolerances when manufacturing that type of web flexure.